Where to put freewheeling diode in relay circuit?

Question

I am building a circuit to lock/unlock a door using a standard electromagnetic door lock (http://www.amazon.com/Seco-Larm-E-941SA-600-Enforcer-Electromagnetic-Holding/dp/B00196YB5Y) and controlling it with a PIC microcontroller. I selected an appropriate relay (http://www.digikey.com/short/7cdzzv) with a 13mA switching current that the PIC could drive safely.

I have read in a number of places that I need a freewheeling diode to protect against inductive loads, and as such I purchased a few 1N4007. My understanding is that I need one across the coil to protect the PIC, but I am wondering since the electromagnet is an inductive load, do I also need one to protect the relay? Most articles talk about the freewheeling diode in one place, but not the other.

Thanks!

Answer 1

Using another diode to protect the relay won't hurt, so I would do it :-)

It's probably not strictly necessary; mechanical switches aren't as sensitive as semiconductors. But without it, there may be sparks inside the relay as it turns off, which will slowly do damage to the contact surfaces.

Answer 2

Connect the cathode to the positive terminal of the coil of the relais and the anode to the negative terminal (GND). You can also use a TVS diode which is unipolar.

Answer 3

You have to use freewheeling diode at the load parallel to it. The diode must be in reverse bias, as the load can get voltage as usual. Otherwise load will get Vd (diode drop. for 1N4007 it is 0.7V)